In 5e D&D, the Suggestion-style spells can suggest a path of action via charm-like forces. This suggests (ha ha) that it alters the thought and feeling process of the target. Obviously the ramifications for life-coaching are astounding, but let us put that aside for the moment.
Much of society &/or culture spends a vast amount on chemicals that do little more than alter state of being (alcohol, cigarettes, coffee are only the socially-expected 'tip of the iceberg'). Indeed, 5e D&D has spells that change the minds of people – but it does not explicitly explain how this changes the impacted person's feelings. For example, a depressed person experiencing Charm is still depressed, but they feel a bit more friendly towards a specific person should they fail their saving throw.
If there is a RAW explanation or example for how (Mass) Suggestion spells &/or effects change feelings beyond thought-action mechanics it changes how the spell impacts the game. Imagine marketing this suggestion-magic as a drug, a valuable resource with NO withdrawal – and minimal risk of addiction. An entire industry would logically exist for any bard, wizard or sorcerer with access to this second level spell.
What would be even more interesting is if this feeling-manipulation could be used to solve psychological barriers. Imagine telling a target 'You will figure out the inner-dialogue on how to be confident… and free of anxiety… in the next two to eight hours.' If psychological states can be over-suggested (meta-suggested?) then this suggestion-impact could even allow a fresh saving throw for all mind-impacting spells.
-
Fear &/ or frightened condition: 'You are NOT afraid! Your course of action is to stop trembling and function normally.'
-
Charmed &/or effects: 'You do not find that vampire charming.'
-
Possession: 'You will cast out that ghost – they are not you! Resist!'
-
Song of Harpies: 'This is not your kind of music, you prefer violin concertos. Do not listen to her!'
At higher levels of Mass Suggestion this would function without concentration. If this mental suggestion-force were allowed it would provide resistance or even immunity to specific magic &/or spells pending the wording of the original (suggest) spell.
In short: Can Suggestion-style magic create specific states of being whilst suggesting an action? Is feeling a state of being considered an action? If so, does this extend into saving throws and resistances against other mind-controlling magic?
Best Answer
Suggestion can't beat condition mechanics
In a general sense if someone is frightened of heights or charmed by a handsome man, you could use Suggestion to prevent that from happening. Would they still feel just as afraid but magically have the strength to face their fear? Good question, I don't have an answer for that, but they may certainly be able to "stop trembling and function normally".
However neither of those things would actually apply the Frightened or Charmed conditions. There is a distinction between being frightened in the normal sense, and being under the effect of the mechanical Frightened condition.
Saving throws represent the target's ability to resist an effect
Remember that usually there is some effect in the game world causing the condition, such as Fear or Charm Person. Your ability to "snap out of it" is the saving throw. The target of Suggestion performs the task "to the best of its ability". If the target fails the saving throw then that is the best of their ability.
DM rulings
In 5e the DM is given a lot of latitude and power when deciding what happens in specific circumstances. It would be entirely reasonable to rule that the target which was suggested "You are NOT afraid!" has Advantage on checks against fear.